1. Field of the Invention
The present invention relates to a fixing apparatus incorporated in an image information recording apparatus such as copying machines, printers, and facsimile machines, and is used for heating an unfixed image on a recording medium. More particularly, the present invention relates to an improvement to a belt-nip type fixing apparatus in which an endless belt is in pressure contact with a heat roller and a recording medium passes through a nip formed between the endless belt and the heat roller.
2. Description of the Related Art
A commonly known conventional fixing apparatus for use in an image forming apparatus usually includes a heat roller having a built-in heater and a pressure roller that rotates in pressure contact with the heat roller. A recording medium carries an unfixed toner image thereon and passes through a nip formed between the heat roller and the pressure roller.
A fixing unit that employs a heat roller requires a certain amount of heat in order to fix toner. When this type of fixing apparatus is used in a color image forming apparatus, the amount of heat required is larger in color image formation than in monochrome image formation.
For color image forming apparatus, the amount of heat required per unit time is larger in high-speed image formation than in low-speed image formation.
The amount of heat supplied to a recording medium during fixing is determined by the following factors:                (1) work of a heat source (wattage=amount of heat/time length),        (2) width of a nip (a dimension of a nip in a direction of travel of a recording medium), and        (3) time length during which the heat roller is in pressure contact with the recording medium.        
In order to increase the amount of heat supplied to the toner deposited on the recording medium, it is required to increase the wattage of a heat source, the width of a nip, or the time length during which the heat roller is in pressure contact with the recording medium. In order to fuse the toner at higher speed, it is necessary to increase the wattage of a heat source or the width of a nip because the heat roller is in pressure contact with the recording medium in a short time length. Because the heat resistance of structural members and a requirement for low power consumption place limitations on the increase in the wattage of a heat source, the wattage cannot be increased beyond a certain limit. Increasing the width of a nip between the heat roller and the pressure roller is a key factor. In order to increase the width of nip, a belt nip type fixing apparatus that employs a belt has been proposed.
FIG. 29 illustrates one such conventional belt-nip type fixing apparatus disclosed in Japanese Patent Laid-Open No. 11-2979. Referring to FIG. 29 a heat roller 101 incorporates a heat source and is rotatable. An endless belt 102 is in pressure contact with the heat roller 101 and is driven by the heat roller 101 in rotation. The endless belt 102 is entrained about a pressure roller 103 so that the endless belt 102 can rotate about the pressure roller 103. The pressure roller 103 is urged by an urging member 104, which in turn urges the endless belt 102 against the heat roller 101. The endless belt 102 is also entrained about a steering roller 105, which serves to eliminate skew of the endless belt 102. The endless belt 102 is also entrained about a support roller 106 urged by urging member 107, which urges the endless belt 102 in an opposite direction to the direction of travel of the recording medium P to apply tension to the endless belt 102. An urging member 109 urges the pressure pad 108 toward the heat roller 101, which in turn urges the endless belt 102 against the heat roller 101 from inside to increase the width of a nip formed between the heat roller 101 and the endless belt 102.
With the aforementioned fixing apparatus, the endless belt 102 needs to be stretched by a predetermined length to ensure that the endless belt 102 is entrained properly. Stretching the endless belt 102 causes more heat to be lost to the environment due to the fact that the endless belt 102 will have a larger surface area except for the nip region. In other words, the endless belt 102 is apt to become cool, allowing more heat to be transferred from the heat roller 101 to the endless belt 102 by conduction. As a result, the heat roller 101 loses a larger amount of heat, thus requiring a longer warming-up time for the fixing apparatus to become ready for fixing.